Abstract: In the crude cellulase system of Trihcoderma reesei, three main enzyme groups involving endo-glucanase (CMCase) , cellobiohydrolase (CBH) and β-Glucosidase were rapidly and selectively denatured in the alkali circumstance. CMCase and CBH were inclined to reversible denaturation, different from the irreversible denaturation of β-Glucosidase as the crude cellulase was incubated statically in the alkali solution at pH9. 00 and (25±1) ℃ for 30min. The residual enzyme activity value of three enzyme groups were respectively remained 58. 8% of CMCase, 56. 6% of CBH and 5. 7% of β-Glucosidase, thus the relative ratio between CMCase or CBH and β-Glucosidase reached 10. 3 or 9. 9. Alkali treatment improved greatly, the cellulase selectivity and produced the glucosidase-poor cellulase to hydrolyze selectively cellulose into cellooligosaccharides containing mainly cellobiose and little cellotriose. The product yield of cellooligosaccharide could reach 6. 73% when 10g/L paper powder solution was hydrolyzed at the enzyme loading of 0. 1% (v/v) alkali treated cellulase for 24h. The oligosaccharides content accounted for 78. 2% of the total degraded saccharides which increased by 53. 6%, compared with the enzymatic hydrolysate with crude cellulase.